1. Field of the Invention
The present invention relates to a method of repairing a pattern using a photomask pattern repair device capable of eliminating a drawback caused when a reference point for detecting an amount of drift of the orbit of a charged particle beam is confirmed.
2. Description of the Related Art
A prior art method of repairing a pattern using a photomask pattern repair device, including a process of forming a glass mask, will now be described, with reference to the accompanying drawings.
FIG. 1 is a plan view showing the whole of a mask pattern and an enlarged view showing a part thereof.
A mask 19, shown in FIG. 1, is obtained by applying a thin film, which contains chromium as the principal ingredient and serves as a shield member, to a quartz glass substrate (not shown) by sputtering or the like.
For example, an EB(Electron Beam) resist is uniformly applied onto the mask 19 and dehydrated. A pattern is drawn on the resist by using an EB pattern drawing apparatus in accordance with pattern data, and then developed. Using the pattern formed on the resist as a mask, the metallic thin film is etched.
After the pattern is completed by the etching of the metallic thin film, the resist is removed, and the resultant structure is washed. After that, registration and critical dimension are measured, and the shape and appearance of a defect in the pattern are inspected. It is only the wafers passed in these measurements and inspections that are placed in a stepper for exposure.
A main pattern section 20, enlarged in FIG. 1, includes a glass substrate 7 and a pattern 5 formed of chromium on the substrate 7 and having a clear defective portion 2. The clear defective portion 2 is detected through a defect shape inspection and repaired by using a carbon or metallic thin film.
The conventional method of repairing a defect in pattern is classified roughly into the following three methods: a lift-off method, a laser repair using a laser beam, and an FIB (Focus Ion Beam) pattern repair using a focus ion beam. The FIB pattern repair includes a method of removing a defect in pattern by etching using an ion beam and a method of repairing a defect in pattern by forming a light shield film so as to cover the defect which is sprayed with a deposition gas and irradiated with an ion beam.
A generally-known FIB pattern repair device is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publications No. 58-56332. One example of the FIB pattern repair device is shown in FIG. 2.
When a pattern formed on a glass substrate 7 placed on a stage 18 is in need of repair, an ion beam 6 is emitted from an ion source 11 and focused upon the substrate 7 through a limiting aperture 12, thereby removing or repairing a defect in the pattern. The focus position of the ion beam can be arbitrarily changed by deflecting the orbit thereof by means of a deflector 14.
A gas gun 15 is provided to generate gas when a protection film serving as a light shield film is formed on a Cr (chromium) film of a pattern member. A Cr secondary ion detector 16 detects a Cr secondary ion 13 generated from the Cr film while a charged particle beam is scanning. An Si secondary ion detector 17 detects an Si secondary ion 13 generated from the glass substrate 7 which is an underlying member of a through hole formed in the protection film and corresponding to a reference point.
The pattern repair using the conventional FIB pattern repair device mentioned above, is performed as follows.
FIG. 3 is a view of a portion of the main pattern section 20 shown in FIG. 1.
In order to repair the defective portion 2 of the pattern member 5 constituted by the Cr film, the portion 2 is irradiated with an ion beam and then sprayed with a gas from the gas gun 15, thereby forming a repairing carbon film 1 covering the defective portion 2.
However, the use of the ion beam sometimes causes a beam drift and makes the beam unstable since the beam source is thermally changed.
The shift in orbit of the ion beam is corrected by detecting a position of the beam with respect to the reference point and changing the orbit based on the mount of drift corresponding to the detected position. According to this correction, which is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 1-309245, a hole is formed, by the ion beam, as a reference point 3 in that part of the pattern member 5 which is in the vicinity of the defective portion 2. The hole is usually formed such that it penetrates the pattern member and reaches the glass substrate.
The repairing carbon film 1 as a repairing area is formed around the defective portion 2 on the basis of the reference point 3 to start repairing the defective portion 2. In order to confirm whether the orbit of an ion beam varies or not during the repair, it is confirmed whether a distance from the orbit position of the currently emitting ion beam to the reference point 3 is maintained or not. A reference point confirmation area 4 is scanned with the ion beam at regular intervals to confirm that the reference point 3 is located in the center of the area 4.
FIG. 4 is an enlarged plan view showing the reference point 3 and reference point confirmation area 4 formed in the pattern member 5, and FIG. 5 is a cross-sectional view taken along the line V--V of FIG. 4.
The reference point confirmation area 4 is scanned with the ion beam at regular intervals of, e.g., a few minutes to several tens of minutes. If the reference point 3 falls within the scanning range, an Si secondary ion having a steep waveform, as shown in FIG. 6, is generated from only the hole of the reference point 3, thereby confirming the position of the reference point.
If the reference point 3 is shifted from the center of the reference confirmation area 4, the shift is corrected as an amount of drift correction. This correction operation is repeated at regular intervals during the repairing operation.
However, the conventional pattern repairing method described above, has the following drawback.
To confirm the position of the reference point 3, the reference point confirmation area 4 is repeatedly scanned several times by several times at regular intervals and, consequently, the Cr film of the reference point confirmation area 4, shown in FIG. 5 is extremely thinned. If, therefore, the number of scans is large, a light leak causes when the pattern is transferred to a wafer, and a new defect is likely to occur on the wafer.
FIGS. 7 and 8 show an example wherein, even if the defective portion 2 of the pattern member 5 is repaired by the repairing carbon film 1, a new defective portion 23 occurs on the repaired transfer pattern 21 since the Cr film of the reference point confirmation area 4 is thinned by scanning the area 4 with the charged particle beam.
The reference point 3, which is not currently in question, is transferred onto the wafer as a pattern, indicated by reference numeral 22 in FIG. 8, as the limit of transfer to the wafer (the resolution of resist) improves in future, with the result that the reference point itself is likely to be formed as a new defect.
In order to eliminate the above drawback, a method of limiting the total number of scans for confirming the reference point is adopted. However, when the reference point is difficult to confirm, the total number of scans reaches a predetermined limit before the repair is completed, and the original repairing operation cannot be continued any longer.
If there is no pattern member of a Cr film for forming a hole serving as the reference point 3 in the vicinity of a defective portion, the foregoing repairing method cannot be employed.
In the prior art repairing method described above, the mass of the Si secondary ion, which is generated as a reference signal from the hole of the reference point 3, is lighter than that of Cr. If foreign substances 8 each having a mass similar to that of Si is present near the reference point 3 as shown in FIG. 9, other secondary ions due to the substances 8 are detected even at positions 9 other than the reference point 3 as shown in FIG. 10, with the result that the correct position of the reference point 3 cannot be determined.
As described above, the prior art repairing method has the drawback wherein a different secondary ion, which has a mass similar to that of Si and is unstable as a reference signal, is easy to be detected by mistake.